The following relates to electronic ballasts. It finds particular application in conjunction with the high intensity discharge lamps (HID), and will be described with particular reference thereto. However, it is to be appreciated that the following is also amenable to other electronically ballasted lamps such as fluorescent lamps and the like.
A ballast is an electrical device which is used to provide power to a load, such as an electrical discharge lamp, and to regulate its current. The ballast provides high voltage to start a lamp, causing the gas to ionize which begins the process of arc formation. Once the arc is established, the ballast allows the lamp to continue to operate by providing proper controlled current flow to the lamp.
Typically, low frequency, square wave ballasts include a three stage power conversion process. Initially, at stage 1, the AC power line voltage is rectified and filtered. At the intermediate stage 2, the DC voltage is converted to the DC current. At stage 3, the DC current is converted to an AC current by an inverter to drive the resonant circuit which excites the lamp. The AC power input voltage is rectified through the full bridge and filtered through the capacitors. The filter typically includes an energy storage capacitor. Since the capacitors do not filter perfectly, the output DC voltage includes an AC component or ripple voltage. The low frequency square wave ballasts do not always reject the AC component of the power line voltage. As a result, the AC ripple propagates to the ballast output and causes undesirable lamp modulation.
One approach to minimize the AC component or ripple of the DC voltage is to use a relatively large capacitance which has a large energy storage capacity. However, the larger capacitors are expensive and occupy more space in the ballast. Additionally, such capacitors are charging only during a short period of time, when the AC component is close to its peak value. The large amount of current is drawn during these short periods of time, causing undesirable harmonics and harmonics distortion in the ballast output waveform.
The present application contemplates new methods and apparatuses that overcome above referenced problems and others.